1. Field of the Invention
This invention relates to a tuner for receiving an input signal and supplying an output signal, comprising and input amplifier, an mixer coupled to the input amplifier for supplying an IF signal, and a multi-stage amplifier for amplifying the IF signal, the multi-stage amplifier comprising a series arrangement of at least two gain stages, and supplying the output signal, and gain control means for controlling the multistage amplifier.
The invention also relates to a cascaded current comparator for use in a multistage amplifier.
2. Description of the Related Art
Such a tuner is known from U.S. Pat. No. 5,210,504. Therein, a tuner is described for satellite broadcasting.
Such a tuner is also known from the article xe2x80x9cMonolithic TV IF system TBA 440xe2x80x9d, E. Schatter, IEEE Transactions on BTR, Vol. 18, August 1972, pages 158-163. In the above-mentioned article, an (IF) amplifier has been described comprising a series arrangement of three gain stages.
A disadvantage of the prior-art tuner is that such a tuner adds noise and distortion to the input signal, especially at a low gain with several gain stages in series.
An object of the invention is to overcome the disadvantages of the prior-art tuner.
To this end, a first aspect of the invention provides a tuner as described above, characterized in that the multistage amplifier further comprises switching means parallel to the gain stages for shunting at least one of the gain stage.
A second aspect of the invention provides a multi-stage amplifier for use in a tuner as described above. By shunting the unnecessary gain stage/stages, the problem of the tuners and/or amplifiers of the prior art are overcome.
A third aspect of the invention provides a cascaded current comparator for use in a multi-stage amplifier as described above, in which inactive stages are turned off. By turning off inactive stages, current dissipation is reduced resulting in a low-power cascaded current comparator.
An embodiment of a tuner according to the invention in which the multi-stage amplifier comprises at least three gain stages arranged in a series.
By using three gain stages the necessary gain range can be easily obtained, and at the same time the complexity of the amplifier is kept within reasonable proportions.
An embodiment of a tuner according to the invention, characterized in that the switching means comprises a switch coupled in parallel with at least two gain stages.
By short-circuiting at least two gain stages together for the low gain range the distortion is further reduced.
An embodiment of a multistage amplifier according to the invention is characterized in that the multistage amplifier comprises a cascaded current comparator for generating tuning signal for the gain stages.
In this way a so-called thermometer code is obtained for supplying the tuning voltages of the gain stages.